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CHAPTER

13 Principles of Ecology

Review Academic VocabularyWrite the correct word for each definition.

experiment organism observation population

1. : hypothesis is tested under controlled conditions

2. : using the senses to study the world

3. : all individuals of one species living in a certain area

4. : any individual living thing

Preview Biology VocabularySee how many key terms from this chapter you already know. Rewrite each phrase, using a different word or words for the words in bold.

PHRASE REWRITTEN WITH DIFFERENT WORDS1. Rocks and air are examples of abiotic

factors.Rocks and air are examples of

.

2. A plant is a producer. A plant .

3. A rabbit is an herbivore. A rabbit is .

Preview Key Concepts 13.1 Ecologists Study Relationships

Ecology is the study of the relationships among organisms and their environments.

13.2 Biotic and Abiotic FactorsEvery ecosystem includes both living and nonliving factors.

13.3 Energy in EcosystemsLife in an ecosystem requires a source of energy.

13.4 Food Chains and Food WebsFood chains and food webs model the flow of energy in an ecosystem.

13.5 Cycling of MatterMatter cycles in and out of an ecosystem.

13.6 Pyramid ModelsPyramids model the distribution of energy and matter in an ecosystem.

GETTING READY TO LEARN

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SECTION

13.1 Ecologists Study RelationshipsKEY CONCEPT Ecology is the study of the relationships among organisms and their environment.

Student text pages 396–400

LEVELS OF ORGANIZATION

The Florida Everglades is an example of the subtropical savanna biome. Many organisms live in this aquatic ecosystem.

Organism

Population

Community

Ecosystem

OrganismEcologists study relationships at different levels of organization.

Ecology is the study of the interactions among living things, and between living things and their surroundings. Ecologists study nature on different levels, from a local to a global scale.

• Organism An organism is one individual living thing.• Population A population is a group of the same species that lives in

one area. • Community A community is a group of different species that live

together in one area. • Ecosystem An ecosystem includes all of the organisms as well as the

climate, soil, water, rocks, and other nonliving things in an area. An entire ecosystem may live within a single decaying log. But the log may be part of a larger ecosystem, such as a forest.

• Biome A biome (BY-OHM) is a major regional or global community of organisms. A biome is usually defined by the climate and by the plant communities that live in an area.

Biome

Ecosystem

Community

Population

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Ecologists study relationships within and between each level of organiza-tion. For example, one ecologist might study a single population of Pacific salmon. Another ecologist might study how the current loss of Pacific salmon affects other species. Yet another ecologist might study the effects of this loss on a global scale.

What are five different levels of organization at which an ecologist might study biological relationships?

Ecological research methods include observation, experimentation, and modeling.

Ecological research involves many different methods and tools. Three main components of ecological research are observation, experimenta-tion, and modeling.

ObservationObservation is the act of carefully watching something over time. Observation may be part of short-term or long-term studies. Long-term studies are important because many environmental changes happen slowly over time.

Observation can involve directly watching populations of organisms. It can also involve indirectly observing populations by tracing signs of the organism’s presence, such as tracks. Scientists also use radio collars to follow the movements of some animals. These collars allow scientists to track organ-isms that move long distances, such as coyotes.

ExperimentationScientists may perform experiments in a lab or in the natural area where the organisms live. There are benefits and draw-backs to each type of experiment. A lab experiment allows for more control of variables. But at the same time, it does not include the complex interactions that happen in nature. An experiment in a natural setting gives a better picture of how organisms really interact. However, a natural setting also makes it difficult to identify the effects of individual variables.

Observation involves carefully watch-ing something over a certain period of time.

McDougal Littell Biology222

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ModelingSome questions cannot be easily investigated through observations or experiments. Models can be used to explore organisms and whole eco-systems in ways that would not be possible in a natural setting. One way of thinking about modeling is that it can be used to answer questions that start with “What would happen if…?”

Models use real data to predict outcomes for different situations. For example, a wide variety of data was combined into a model of Yellow-stone National Park. This model was then used to decide how best to reintroduce grey wolves into the park.

What are three components of ecological research?

ecology ecosystem

community biome

Fill in the blanks with the correct term from the list above.

1. is the study of interactions among living things, and between living things and their surroundings.

2. A major regional or global community of organisms is a(n) .

3. A(n) is all of the living things as well as the nonliving things in an area.

4. A group of different species that all live together in one area is a(n) .

5. Put the following in order from largest to smallest level of organization: community, biome, population, individual, ecosystem.

6. Describe the benefi ts and drawbacks of experiments in a laboratory compared with experiments in a natural setting.

13.1 Vocabulary Check

Go back and highlight each sentence that has a vocabulary word in bold.

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13.1 The Big Picture


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SECTION

13.2

An ecosystem includes both biotic and abiotic factors.

All ecosystems are made up of living and nonliving components.* These parts are called biotic and abiotic factors.

• Biotic (by-AHT-ihk) factors are living things, such as plants, animals, fungi, and bacteria.

• Abiotic (AY-by-AHT-ihk) factors are nonliving things, such as tem-perature, moisture, wind, rocks, and sunlight.

In an ecosystem, biotic and abiotic factors work together in a complex web.

Give one example of a biotic factor and one example of an abiotic factor.

Changing one factor in an ecosystem can affect many other factors.

Every organism depends on a combination of biotic and abiotic resources to live. All species—including humans—are affected by changes to the biotic and abiotic factors in an ecosystem.

Biodiversity (BY-oh-dih-VUR-sih-tee) is the variety of living things in an ecosystem. Some areas of Earth, including tropical rain forests, have very high biodiversity. Tropical rain forests cover less than 7 percent of Earth’s ground surface, but contain over 50 percent of Earth’s plant and animal species.

A single change in an ecosystem can have a variety of effects. Some changes may have very little effect. But in some cases, the loss of one species can have a very large effect on an entire ecosystem. A keystone species is a species that has an unusually large effect on its ecosystem.

Biotic and Abiotic FactorsKEY CONCEPT Every ecosystem includes both living and nonliving factors.

VISUAL VOCAB

Like a keystone that holds up an arch, a keystone species holds together a dynamic ecosystem.

keystone

* ACADEMIC VOCABULARY

component part


Give one example of a biotic factor and one example of an abiotic factor.

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One example of a keystone species is the beaver. Beavers cut down trees and build dams on rivers. This changes the ecosystem by turning a river into areas of ponds and marshes. Many different species can live in the pond and marsh environments that are made by beavers.

What do beavers make that results in a wetland ecosystem?

biotic biodiversity

abiotic keystone species

Circle the correct term from each pair to complete the sentences below.

1. Wind and rocks are two examples of biotic / abiotic factors.

2. Fungi and plants are two examples of biotic / abiotic factors.

3. A keystone species has a large / small effect on an ecosystem.

4. Biodiversity is the quantity / variety of organisms in an area.

5. What are two categories of factors that make up an ecosystem?



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KEYSTONE SPECIES

increased fish population

increased waterfowl population

creation of wetland

ecosystem

nesting sites for birds

keystone species


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SECTION

13.3Producers provide energy for other organisms in an ecosystem.

In Section 13.2 you read that ecosystems are made up of both biotic and abiotic factors. Another important part of an ecosystem is the flow of energy. All organisms need a source of energy in order to survive.

• Producers are organisms that make their own food. Plants and other photosynthesizing organisms are producers. Producers are also called autotrophs.

• Consumers are organisms that get their energy by eating other organisms, including plants and animals. Consumers are also called heterotrophs.

Producers provide the basis for an ecosystem’s energy. Some consumers eat producers. For example, moose and elk eat plants. Some consumers eat other consumers. For example, the grey wolf eats moose and elk. But all consumers depend on producers. Without producers, moose and elk could not survive and without moose and elk, the wolf could not survive.

Energy in EcosystemsKEY CONCEPT Life in an ecosystem requires a source of energy.

Plants are producers, or autotrophs. Animals are consumers, or heterotrophs.

Circle the names of three consumers in the paragraph above.

The suffix –troph comes from a Greek word mean-ing “nourishment,” or food. • The prefix auto– means

“self.” • The prefix hetero–

means “different.”

VOCABULARY

Student text pages 40–43Student text pages 40–43


Circle the names of three consumers in the paragraph above.

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Almost all producers obtain energy from sunlight.

Most producers on Earth use sunlight as their energy source. Photosyn-thesis is the process by which plants and some protists, such as green algae, use energy from the Sun to make sugars. Plants use these sugars as energy for cellular respiration.

Not all producers rely on the Sun for energy. In 1977 scientists first visited deep-sea vents on the floor of the ocean. They were very sur-prised to find many different organisms living in an ocean floor ecosys-tem, far from the reach of sunlight.

The producers in this ocean floor ecosystem are prokaryotes that make their own food, using chemicals as an energy source, not the Sun. This process is called chemosynthesis (KEE-moh-SIHN-thih-sihs). Chemosynthetic organisms also live in hot springs.

Underline two places that chemosynthetic organisms live.

producer heterotroph

autotroph chemosynthesis

consumer

Fill in the blanks with the correct term from the list above.

1. Which two words describe an organism that eats other organisms as food?

2. Which two words describe an organism that makes its own food?

3. List two different energy sources for producers. Circle the one that is the source for most producers.



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A food chain is a model that shows a sequence of feeding relationships.

As you have read, energy flows through an ecosystem from producers to consumers. A simple way to represent this flow of energy is with a food chain. A food chain shows the feeding relationships for a single chain of producers and consumers.

Types of ConsumersAs you can see in the food chain above, not all consumers are alike. Different types of consumers have different food sources.

• Herbivores, such as the rabbit above, are organisms that eat only plants.

• Carnivores, such as the hawk above, are organisms that eat only animals.

• Omnivores are organisms that eat both plants and animals. Most humans are omnivores.

• Detritivores (dih-TRY-tuh-VOHRZ) are organisms that eat dead plant and animal matter. Earthworms, for example, are detritivores.

• Decomposers are detritivores that break down plant and animal matter into simpler compounds. Fungi, for example, are decomposers. Decomposers return nutrients to the ecosystem.

Food Chains and Food WebsKEY CONCEPT Food chains and food webs model the flow of energy in an ecosystem.

SECTION

13.4

Grasses are producers. They get energy through photosynthesis.

Rabbits are consumers. They eat producers, such as grasses.

Hawks are also consumers. They eat other consumers, such as rabbits.


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Some organisms eat only one or a few specific types of organisms. For example, a bird called the Florida snail kite eats mostly one particular type of snail. Organisms that have a very selective diet are called specialists. Because specialists eat only one or a few particular organisms, they are very sensitive to changes in the populations of organisms they eat. For example, if the snail population drops, the Florida snail kite does not have another main food source.

Other organisms, called generalists, eat a variety of differ-ent organisms. For example, the grey wolf eats many different animals, including elk, moose, deer, beavers, and mice.

Trophic LevelsThe figure on page 228 shows a food chain of grasses (producers)—rabbit (herbivore)—hawk (carnivore). You can think of each link in a food chain as a level of feeding, or a trophic level. Energy flows up the food chain from the lowest trophic level to the highest.

• Producers are the first, or bottom, trophic level. • The next trophic level is made of primary consumers—herbivores

that eat producers.• The next trophic level is made of secondary consumers—carnivores

that eat herbivores. • Continuing up the food chain, tertiary consumers are carnivores that

eat secondary consumers. Omnivores, such as most humans, can be listed at different trophic levels in different food chains. A person is at the level of primary consumer when eating vegetables. A person is at the level of secondary consumer when eating beef or chicken.

At what trophic level are herbivores found?

A food web shows a complex network of feeding relationships.

A food chain shows a simple sequence of feeding relationships. But most feeding relationships are not very simple. For example, a generalist such as the grey wolf may be a part of several food chains that involve elk, deer, mice, and other organisms. This complex network of feeding relationships and the related flow of energy can be represented by a food web.

Primary means first in order.Secondary means second in order. Tertiary means third in order.

VOCABULARY

See food web in student text, pg. 410

Visual Connection

A bird called the Florida snail kite is a specialist. It eats mostly one type of snail, like the organism shown above.


At what trophic level are herbivores found?

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The stability of a food web depends on producers. Notice that the feed-ing chains of all organisms can be traced back to producers. In the food web above, a variety of prairie plants are the base of the food web.

Also notice that some organisms can be involved with the food web at different trophic levels, depending on what they eat. When the hawk eats a mouse, it is a secondary consumer. But when it eats a snake—that ate a beetle that ate plants—it is a tertiary consumer. At each link in a food web, some energy is stored within an organism but most energy is lost to the environment as heat.

In the food web shown above, at what trophic level does the spider feed?

FOOD WEB

This fox is a secondary consumer and gets its energy by eating rabbits, squirrels, mice, or sparrows.

This rabbit is a primary consumer and gets its energy by eating plants.

A food web shows the network of feeding relationships between trophic levels in an ecosystem. Food webs can be quite complex, because many organisms feed on a variety of other species.

Plants are producers and get their energy from the Sun.

This hawk may be a secondary consumer, a tertiary consumer, or even at a higher trophic level, depending on what it eats.


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food chain decomposer

herbivore specialist

carnivore generalist

omnivore trophic level

detritivore food web

1. What is the difference between a food chain and a food web?

2. What is the difference between a specialist and a generalist?

3. What is the difference between a detritivore and a decomposer?

4. Fill in the chart below to describe your place in the food web.

LIST THE LAST THREE TYPES OF FOOD THAT YOU ATE.

WHAT TYPE OF CONSUMER WERE YOU?

AT WHAT TROPHIC LEVEL DID YOU EAT?



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The hydrologic cycle is the circular pathway of water on Earth.

Water cycles through the environment. Water moves continuously through the water cycle. The water cycle, or the hydrologic cycle (HY-druh-LAHJ-ihk), is the circular pathway of water on Earth—from the atmosphere, to the surface, below ground, and back into the atmosphere again. On Earth’s surface, living things—including you—are part of the water cycle.

Cycling of MatterKEY CONCEPT Matter cycles in and out of an ecosystem.

SECTION

13.5

HYDROLOGIC CYCLE

precipitationcondensation

transpirationevaporation

water storagein ocean

surfacerunoff

lake

groundwater

seepage

Name two ways that water can enter the atmosphere.


vapor the gaseous state of a substance

During precipitation, water falls to the ground as rain or snow. Water may trickle through the ground in the process of seepage. Liquid water becomes vapor in the process of evaporation. When water evaporates from plants, it’s called transpiration. Water vapor becomes liquid water again during condensation.


B.3.4

Name two ways that water can enter the atmosphere.

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Elements essential* for life also cycle through ecosystems.

Oxygen, carbon, nitrogen, hydrogen, phosphorus, and sulfur are some of the elements necessary for life. Like water, these elements also cycle through ecosystems. The movement of a particular chemical through the living and nonliving parts of an ecosystem is called a biogeochemical cycle (BY-oh-JEE-oh-KHEM-ih-kuhl). Here you will read about four biogeochemical cycles: the oxygen cycle, the carbon cycle, the nitrogen cycle, and the phosphorus cycle.

The Oxygen CycleMost organisms use oxygen for cellular respiration. Recall from Chapter 4 that plants and other photosynthesizing organisms release oxygen as a waste product.

The Carbon CycleCarbon is a main component of carbohydrates, proteins, fats, and all of the other molecules that make up living things. Carbon can be found in many different forms—as gas in the atmosphere, dissolved in water, in fossil fuels such as oil and coal, in rocks such as limestone, and in the soil.

Plants convert carbon dioxide from the air into carbohydrates. Carbo-hydrates get passed through the living world as one organism eats another. Processes such as respiration and the burning of fossil fuels return carbon to the atmosphere.


essential necessary, required for

Carbon dioxide from the atmosphere is used by plants during pho-tosynthesis. Respiration releases carbon dioxide back into the atmo-sphere. The burning of fossil fuels, such as oil and gas, releases carbon dioxide into the atmo-sphere as well. Carbon dioxide also returns to the atmosphere as dead organisms decompose.

CARBON CYCLEcarbondioxide

in air combustion

photosynthesis

carbon dioxide dissolved in water

respiration

decomposition of organisms

photosynthesis

respiration

fossil fuels

In the oxygen cycle, oxygen is produced through photosynthesis. Living organ-isms take in this oxygen and release it as carbon dioxide through respiration.

oxygen

respiration

carbondioxide

photosynthesis

OXYGEN CYCLE


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The Nitrogen CycleAbout 78 percent of the atmosphere is made of nitrogen gas. Organisms need nitrogen to live, but most organisms cannot use nitrogen in a gas form. Instead, most organisms can only use nitrogen when it is in the form of ions such as ammonium (NH4

�) or nitrate (NO3�). Certain

types of bacteria can turn nitrogen gas into ammonia through a process called nitrogen fixation.

Much of the nitrogen cycle happens underground. After nitrogen fixation, other bacteria turn the product, ammonia, into nitrates. Nitrates are used by plants to make amino acids and proteins. Nitrogen moves through the living world as one organism eats another. Some types of bacteria also use nitrates, and release nitrogen gas back into the atmosphere.

NITROGEN CYCLE

nitrogen in atmosphere

nitrogen-fixing bacteria in roots

decomposers

ammonium

nitrifyingbacteria

plants

animals

nitrogen-fixing bacteria in soil

nitrates

nitrites

nitrifyingbacteria

denitrifyingbacteria

ammonification

Nitrogen gas in the atmosphere is changed into ammonia by ni-trogen-fi xing bacteria. Ammonia becomes am-monium, which nitrify-ing bacteria change into nitrates. Plants use nitrates to make amino acids and proteins. This nitrogen is passed through the food web. Denitrifying bacteria change nitrates back into nitrogen gas, which is released into the atmosphere.


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The Phosphorus CycleThe oxygen, carbon, and nitro-gen cycles all have some part that involves atmospheric gases. The phosphorus cycle is different. Most of the phosphorus cycle takes place at ground level.

Phosphate is released by the slow breakdown of rocks. Plants take up phosphate through their roots. Phosphorus then moves through the food web. When dead organisms are broken down by decomposers, phosphorus is released back into the environment.

Underline the main difference between the phosphorus cycle and the other cycles in this section.

hydrologic cycle

biogeochemical cycle

nitrogen fixation

1. List four biogeochemical cycles:

2. Nitrogen fi xation changes into .

3. The hydrologic cycle is the path of what substance?

4. What two main biologcal processes are responsible for the cycling of oxygen?



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PHOSPHORUS CYCLE

geologic upliftingrain

decomposers

plants

animals

sedimentation forms new rocks

phosphate in solution

leaching

phosphate in soil

runoff

weathering of phosphate from rocks

The weathering of rocks releases phosphates into soil and water. Plants take up phosphates, which are then passed through the food web. Phophates are released back into the soil when these organisms die. Some phosphates sink to the botttom of water bodies, where they may become rock over thousands of years.


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An energy pyramid shows the distribution of energy among trophic levels.

Producers use energy from sunlight to make food. Herbivores eat plants—the producers—to get energy. Some of the energy is used by the animals to grow and some is used for cellular respiration. However, most of the energy that is consumed is lost as heat. Carnivores then eat the herbivores. And again, most of the energy is lost as heat.

An energy pyramid is a diagram that compares the energy used by producers, primary consumers, and other trophic levels. In other words, an energy pyramid shows how much energy is available at each trophic level. Energy is lost at each trophic level of a food chain. Because of this, a typical energy pyramid has a large base of producers. Each level above gets smaller, because as energy is lost as heat, there is less energy avail-able as food for organisms. The longer the food chain, the more energy is lost between the bottom and top links.

Pyramid ModelsKEY CONCEPT Pyramids model the distribution of energy and matter in an ecosystem.

SECTION

13.6

energylost as heat

energy transferred

Where are producers located on the energy pyramid?

An energy pyramid shows the energy fl ow between trophic levels in an eco-system. Between each level, up to 90 percent of the energy is lost as heat into the environment.


B.3.5

Where are producers located on the energy pyramid?

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Other pyramid models illustrate an ecosystem’s biomass and distribution of organisms.

An energy pyramid shows energy loss at each trophic level. Pyramid diagrams can also be used to represent other components of an ecosys-tem. Two other types of pyramid models are a biomass pyramid and a pyramid of numbers.

Biomass PyramidsBiomass is a measure of the total amount, or dry mass, of organisms in a given area. A biomass pyramid is a diagram that compares the biomass of different trophic levels within an ecosystem. It shows the mass of producers that are needed to support primary consumers, the mass of primary consumers required to support secondary consumers, and so on. Notice that each trophic level has a smaller biomass than the one below it.

The prefix bio- refers to life or living things. Mass is a measurement of the amount of matter. Biomass is a measure-ment of the amount of living organisms in an area.

VOCABULARY

The biomass pyramid shows the total dry mass of organisms found at each trophic level.

primaryconsumers

secondaryconsumers

tertiaryconsumers

675 g/m2

150 g/m2

75 g/m2

producers 2000 g/m2


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Pyramids of NumbersA pyramid of numbers gives a count of the numbers of individual organ-isms at each trophic level in an ecosystem. This type of pyramid gives a good picture of the large numbers of producers that are required to support just a few top-level consumers. Notice that it takes a huge number of producers to support just a few mountain lions.

What is represented by a pyramid of numbers?

biomass

energy pyramid

1. What is represented by an energy pyramid?

2. What is represented by a biomass pyramid?

3. Which trophic level contains the most energy?

4. What happens to most energy at each trophic level?



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primaryconsumers

secondaryconsumers

tertiaryconsumers

500,000

5000

5

producers

In a pyramid of numbers, each level represents the actual number of organisms at each trophic level.

5,000,000


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Chapter 13 Review 1. What is a food web?

2. Label the energy pyramid with the follow-ing terms: tertiary consumers, producers, secondary consumers, primary consumers.

3. On the energy pyramid, draw an arrow pointing to the trophic level that has the most available energy.

4. The source of energy for most autotrophs is a. chemicals. b. oxygen. c. carbon dioxide. d. sunlight.

5. Explain why the following statement is true: The stability of an eco-system depends on producers.

6. Which trophic level receives the least amount of energy? a. producers b. tertiary consumers c. secondary consumers d. primary consumers

7. Use the following terms to label the drawings below: herbivore, producer, carnivore.

8. How do elements such as carbon and nitrogen get moved through the living world?


B.3.4

B.3.5

B.3.5

B.3.5

B.3.5

B.3.5

B.3.5

B.3.4

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